mpg calculated vs computer

Doesn't this go against warnings not to top off your gas tank? Aren't there mechanisms that can be damaged by overfilling?

 

My Accord manual says - 'Do not top off the tank. This leaves room in the tank for fuel to expand due to varying temperatures.'

I would think that's true for all cars, especially with warnings posted at the fuel stations.

 

mpg displayed 47.2
mpg calculated 47.4 this is for the first tank refill. I only went by the nozzle clicking off, not filling up to the neck of the tank.

 

With just a couple of fill ups I think any variations in pump shut-off will cancel out. I've never been a fan of filling to the neck, for various negative reasons, not the least of which being it takes too dang long, tends to tick off anyone waiting behind you.

Also, Toyota says they are making the computer more "accurate"? IMHO, that's a polite way to put it: why is the inaccuracy consistantly, with every Prius owner, in their favour, by 6~8%?

 

It looks impressive for people who don't actually calculate their real MPG.

 

My last tank, computer indicated 60.2 mpg, calculated is 56.3 mpg. 2010 model III, 45,600 miles.

 

After 2300 miles, approx.700 miles on interstate averaging about 65 to 70 mph, the remaining miles in town or 55 mph traveling. Calculated MPG is 45.3. Displayed MPG is 46.6. So displayed is + 1.2 MPG.

 

I agree with cycledrum. I distinctly remember reading in the owners' manual to NOT top up. There is a flexible bladder in the fuel tank which is supposed to help control fuel fumes. If you top up, you stretch the bladder, causing it to lose its
function.

I stop filling the gas tank when the nozzle switches off. My manual calcs differ from computer shown display of mpg by as much as 3-4 mpg, and then sometimes there's hardly any difference at all. I have no idea why.

 

Your numbers work out to 2.8% error. That's a fair bit less the regular Prius owners are finding. Our error is at 6.4% right now. On the optomistic side, of course.

 

This is a v forum.

 

Yup. The EVAP system, specifically the charcoal canister, is not designed to be filled with liquid gasoline. Overfilling can kill it, throw error codes, and eventually end up in an expensive repair which may not be covered by warranty.

Just fill til the nozzle clicks off the first time (slowing down as you approach full to avoid premature click-offs). That plus making sure the car is level when you fill will be accurate enough.

If you want even greater consistency you can fill at the same station and use the same pump (not always possible, obviously).

 

At 4930 miles, My displayed mpg reads 48.0. The calculated mpg is 45.3. It has been running at this 2.6 / 2.7 mpg difference for the last approx. 2700 miles. This is in FL. weather and flat terain for approx. the last 3000 miles.

 

I have had about 10 fill-ups since getting my Prius v. My driving has not changed during that time. The car's computer estimates I'm averaging about 43 MPG. My own calculations with each fill-up shows that I'm getting about 41 MPG.

 

41.9 for the computer, 38.9 for the fuelly.com calculation. This was only our first fill up in the car though so I expect it will get better